1. Field of the Invention
The present invention relates to power control of a radio base transceiver station set (hereinafter referred to as “base transceiver station”) and a radio terminal set (hereinafter referred to as “mobile station”) and particularly to a power control circuit and a radio transmission apparatus which are suitable for use in a radio system that transmission power is controlled on the basis of access communication quality.
2. Description of the Related Art
In a cellular mobile communication system (radio system) of a wideband-code division multiple access system (hereinafter referred to as “W-CDMA system”), a plurality of mobile stations use carriers (carrier waves) of the same frequency. Accordingly, when the individual mobile stations transmit carriers having identical transmission power irrespective of distance to a base transceiver station, the base transceiver station receives the carrier with a great power value from a mobile station close to the base transceiver station and cannot extract the carrier from a mobile station distant from the base transceiver station to separate it (this is referred to as “near-far problem”).
In order to solve this near-far problem, the cellular mobile communication system executes transmission power control (TPC). The TPC means that a mobile station and a base transceiver station control transmission power necessary for communication between them to the necessary minimum power value. A region for inserting TPC bits (TPC information or TPC bits information) is provided in radio channels of the mobile and base transceiver stations, and both mobile station and base transceiver station extract TPC bits from radio signals transmitted by the mobile station or the base transceiver station to select “rise in transmission power”/“drop in transmission power”.
The transmission power of the base transceiver station is thereby feedback-controlled by the TPC bits transmitted by the mobile station, and the transmission power of the mobile station is also feedback-controlled by the TPC bits transmitted by the base transceiver station. Accordingly, viewed from the whole cellular mobile communication system, the interference by mobile stations in the base transceiver station is reduced, and so the increase of accommodated users (subscribers) is facilitated.
The judgment as to whether the increase or decrease of the transmission power follows TPC or not has heretofore been made by whether the radio channel in a base transceiver station or a mobile station is established synchronously with the base transceiver station or the mobile station or not. The presence of the synchronism establishment in this radio channel has been judged by whether the base transceiver station or the mobile station can exactly detect a pilot symbol contained in a frame received or not. The pilot symbol is contained in a slot allotted to each user, and a different pattern is allotted to each user. For example, when the base transceiver station receives a pilot symbol “101010” (see FIG. 8(c)), the base transceiver station can recognize that this pilot symbol is one allotted to a certain user. More specifically, the base transceiver station can distinguish pilot symbols in 15 slots #0 to #14 from one another and recognize that they are respectively transmitted by 15 mobile stations. The pattern of a pilot symbol is provided in detail every upstream channel (channel from a base transceiver station to a mobile station) and every downstream channel (channel from the mobile station to the base transceiver station) and according to a bit rate or symbol rate of each channel. The number of bits of this pilot symbol is separately provided in the upstream channel and the downstream channel. With respect to, for example, the pilot symbols of the upstream channel, since each slot has pilot bits (bits respectively indicating pilot symbols) of 6 bits, and 1 frame has 15 slots #0 to #14, the base transceiver station receives pilot bits of 90 bits in 1 frame. The base transceiver station can know the number of bits that can be exactly demodulated and the number of bits that cannot be exactly demodulated among 90 bits and judges synchronism establishment or out-of-synchronism on the basis of this number of bits. In this specification, these 90 bits refer to as “synchronization patterns”.
The synchronism detection is used in various setting and mode control in an apparatus, such as on/off control which will be described subsequently, fixing of a window of a searcher and setting of a diversity mode. The on/off control of TPC means control that TPC is made off (not executed) when a base transceiver station and a mobile are out of synchronism, and TPC is made on (executed) when they are synchronized with each other. Control making use of TPC and control making no use of TPC are referred to as “TPC on” and “TPC off”, respectively.
As well known, a protective circuit (not illustrated) is provided for frame synchronism detection. This protective circuit has 2 functions, and a first function is a forward protection function that judges to be in a state out of synchronism when changes in synchronization patterns are continuously monitored, and abnormal patterns have been continuously detected prescribed times (hereinafter referred to as “forward protection”). A second function is a backward protection function that judges to be in a state that frame synchronism has been completed or restored when synchronization patterns have been continuously detected prescribed times (hereinafter referred to as “backward protection”).
The frame synchronism means synchronism every frame, and system synchronism, which will be described subsequently, means a state this frame synchronism continues prescribed times.
FIG. 20 illustrates an example of state transition as to synchronism. A base transceiver station or a mobile station transfers or changes from a state S100 of synchronism establishment to a state S200 of out-of-synchronism when the out-of-synchronism occurs in the state S100 of synchronism establishment as shown in FIG. 20. Forward protection operation is performed so as to exactly conduct this transition. The base transceiver station or the mobile station transfers to the state S100 of synchronism establishment when the synchronism is restored in the state S200 of out-of-synchronism. Backward protection operation is performed for this transition. These functions are exhibited by means of a counter for forward protection and a counter for backward protection, respectively.
As described above, the cellular mobile communication system makes control by transmitting and receiving TPC bits through the up/downstream channel and judges the synchronized state of radio signals using pilot symbols (synchronization patterns), forward protection and backward protection.
On the other hand, the mobile station is also so constituted that synchronism establishment or out-of-synchronism can be judged by demodulating the downstream channel from the base transceiver station to obtain a pilot symbol of the prescribed number of bits.
Such is the description as to the base transceiver station and mobile station in broad outline.
A transmitting and receiving method of TPC bits will hereinafter be described on both upstream and downstream channels. The details thereof are described in the specification of 3GPP (3rd Generation Partnership Project).
With respect to the upstream channel, the mobile station transmits TPC bits and data of I (In-Phase) and Q (Quadrature) to be transmitted using 2 upstream physical channels (channels for transmitting control information mainly produced by a layer 1). The TPC bits are composed of, for example, 2 bits (4, 8 or 16 bits in some cases). When the TPC bits are “11” (“1111”, “11111111” or “1111111111111111”), “rise in transmission power (TPC command: 1)” is represented. When the TPC bits are “00” (“0000”, “00000000” or “0000000000000000”) by way of example, “drop in transmission power (TPC command: 0)” is represented.
The downstream channel will now be described. In the downstream channel as well, 15 slots #0 to #14 respectively correspond to mobile stations, and the length of the slots #0 to #14 is ( 10/15) ms. With respect to the downstream channel, when the whole bit of the TPC bits of 2 bits (4, 8 or 16 bits in some cases) is “1” or “0”, “rise in transmission power (TPC command: 1)” or “drop in transmission power (TPC command: 0)” is represented likewise.
Since the forward protection is used for synchronism detection in the prior art, however, there is a period that asynchronism is not detected even in a state that a synchronization pattern is not exactly recognized. In such a case, there is a high possibility that the TPC bits received by the base transceiver station or mobile station may be erroneously recognized. In addition, since the TPC control is turned on/off by detection of synchronism/asynchronism, erroneous TPC bits transmission control comes to be continued. Thus, an evil that the base transceiver station or mobile station transmits radio signals having a greater power value than the station needs or an excess power value, or that deterioration of quality is advanced has developed.
Further, the base transceiver station or mobile station starts operation of the forward protection when the out-of-synchronism occurs, and continuously transmits a radio signal to the effect that a power value during the operation of the forward protection just before the out-of-synchronism occurs is made a constant power value. In other words, the power value controlled by such erroneous TPC bits as described above is made a constant power value upon asynchronism.
Thus, the base transceiver station or mobile station extracts TPC bits received during the operation of the forward protection when the out-of-synchronism occurs, and the transmission power is not controlled, and transmits a signal, by which a so-called random power value not power-controlled may be taken, with low reliability on the basis of this TPC bits. In other words, there are problems that the base transceiver station or mobile station cannot transmit a signal with the optimum power value, and that the cellular mobile communication system has a possibility that the quality thereof may not be retained.
The base transceiver station or mobile station judges the synchronism establishment and the out-of-synchronism by calculation (mobile calculation or mobile operation) of the error probability of pilot symbols in every frame. Specifically, the base transceiver station or mobile station determines TPC to execute TPC when, for example, at least 5 bits among 90 bits have an error, or not to execute TPC when less than 5 bits have an error. Accordingly, the base transceiver station or mobile station must make a mobile calculation every frame, and deterioration of a receive environment during the mobile calculation cannot be detected, and so a greater power value than the station needs or an excess power value is also used. In other words, there is a problem that the base transceiver station or mobile station cannot exactly stop TPC.